NORTHAMPTON — While lawmakers consider whether to use military force in retaliation against Syria’s alleged use of nerve gas on its citizens, the National Priorities Project has calculated such action may cost hundreds of millions or billions of dollars.

Meanwhile, events are planned throughout the Pioneer Valley to be held in conjunction with a scheduled peace vigil led by Pope Francis in the Vatican on Saturday.

According to the National Priorities Project, a Northampton nonprofit that tracks how federal tax money is spent, the Pentagon is already planning on purchasing 200 Tomahawk missiles this year at a cost of about $320 million.

The National Priorities Project calculates that to work out about $36,000 in tax dollars per hour.

That figure does not take into account the likely increase in the number of missiles that will have to be purchased if the U.S. launches multiple strikes against Syria.

For comparison, U.S. forces launched 110 Tomahawk missiles in the first hour of the strike on Lybia in 2011, according to the National Priorities Project.

How much military action against Syria would ultimately cost is uncertain, based on past experience, National Priorities Project research director Mattea Kramer said in a statement.

“Back in 2003, Bush administration officials projected $60 billion as a high-end estimate for the Iraq war,” Kramer said. “A decade later, the cost of the Iraq war has exceeded $800 billion, including $7 billion this year.”

“Bottom line, right now, it’s impossible to know if military intervention in Syria will cost the U.S. $100 million or hundreds of billions,” Kramer said.

Meanwhile, public support for military action seems to be waning.

According to a recent Reuters poll, 56 percent of those who responded are against U.S. intervention in the embattled country.

The National Priorities Project said polling data shows 40 percent of respondents are in favor of humanitarian aid rather than military strikes.

Vigils planned

According to a statement from the Springfield Diocese, Bishop Timothy A. McDonnell has asked that all parishes that are holding vigil masses in repsonse to the Syria situation to also conduct an hour-long exposition of the Blessed Sacrament and to invite parishioners to pray during that time.

Churches in Hampshire, Hampden, Berkshire and Franklin counties are planning Syria-related vigils at various times throughout the day.

Scheduled vigils in the area follow:

■ St. Elizabeth Ann Seton Parish in Northampton will conduct a vigil after the 4 p.m. Mass from 5 to 6.

■ Our Lady of the Valley in Easthampton will hold an Adoration and Benediction following the 5:30 p.m. Mass.

■ St. Francis Church in Belchertown will conduct an Adoration, Rosary and Mass from 3 to 4 p.m.

■ St. Theresa Church in South Hadley will hold Adoration from 5 to 6 p.m.

■ Immaculate Heart of Mary Church in Granby will conduct an Exposition of the Blessed Sacrament, prayer time and Benediction following the 4 p.m. Mass.

■ St. Mary’s Church in Westfield will have a holy hour following the 4 p.m. Mass.

■ Our Lady of the Blessed Sacrament Church in Westfield will hold a eucharistic holy hour for peace following the 4 p.m. Mass.